SeaTag-6K

An Argos satellite pop-up tag (PSAT) for investigation of marine animals in the bathypelagic and abyssopelagic zones

Odoo CMS - a big picture

Several species have recently been discovered to dive much deeper than previously known. Anecdotal reports from fishermen indicate possible deep diving of Megamouth sharks. (Courtesy Paul Clerkin, Moss Landing Marine Lab)

Last spring I received an urgent message. A number of animals had been tagged with SeaTag-MOD PSAT Argos tags and now several of these tags had popped up, reporting automatic crush depth release at 1850 m. The tagged species (the researcher requested confidentiality) was not known to venture anywhere near that deep. The deep oceans including the bathypelagic and abyssopelagic zones and their vertical connectivity to the surface are a research topic of increasing importance. As a session description for the upcoming ASLO 2017 conference puts it:

Recent studies highlight that: 1) the global biomass of deep-pelagic fishes has been severely underestimated; 2) trophic efficiency through the nekton may be much higher than previously thought; and 3) a significant portion of oceanic autotrophic carbon may be respired at depth by nekton.

Indeed, there is recent evidence that several large species including Cuvier's Beaked Whale (3000 m observed) and whale sharks (1800 m observed) dive far deeper than previously known.

The Design of the SeaTag-6K

This discovery led to the development a SeaTag-MOD variant suitable for investigations in the bathypelagic and abyssopelagic zones, i.e. at depths up to 6000 m.

Odoo CMS - a big picture

A SeaTag-MOD with 2000 m depth rating is crushed during a 6000 m pressure tank ride, demonstrating the extreme forces and identifying vulnerabilities that were addressed in the SeaTag-6K design

The first order of business was to test a regular, 2000 m depth rated SeaTag-MOD at high pressure to identify vulnerabilities. After a 6000 m pressure tank ride, we found the syntactic foam of the battery section, shielded from fluid contact by an outer plastic tube, sucked in on itself, microspheres crushed. The syntactic foam float body meanwhile had crumbled. SeaTag-6K is thus manufactured with a syntactic float body rated to 6842 m/10000 PSI, carbon fiber bracing, depth sensor rated to 10,000 PSI, and replacement or support of components with identified vulnerabilities.

SeaTag-6K Operational Capabilities 

Odoo CMS - a big picture

SeaTag-6K, and Argos Pop-Up Satellite Tag (PSAT) is design pressure tested to a depth of 6842 m of sea water (10,000 PSI). Resistance to these extreme pressures required the use of a heavier syntactic foam composition and therefore a bigger float.

Not unlike any deep ocean submersible, a large amount of heavy syntactic foam is needed to offset weight, making this a large PSAT tag. The tag however is intrinsically safe to the tagged animal or nearby equipment as the solid design has no voids that might implode. Its large 2 GB memory is big enough for any realistic data series.

The SeaTag-6K sensor suite includes depth, temperature, 3-axis magnetometer, 3-axis accelerometer and a wrap-around solar panel doubling as a light sensor. The solar panel has been identified as a point of vulnerability, but is available for limited use (see below).

Odoo CMS - a big picture

Figure 1: Tag S/N 5016 is exposed to progressively greater pressures during design validation. The tag is designed to a 10,000 PSI (6800m) standard.

SeaTag-6K is also capable to provide horizontal migratory tracks. While light does not reach these depths, estimated tracks are available based on magnetic field intensity observations alone. Figure 2 (left) shows the estimated track of a SeaTag-MOD ocean drifter obtained using light, SST and geomagnetic measurements. It closely tracks the Argos reference track (not shown here). Figure 2 (right) is the same track, but generated using geomagnetic measurements only. The much broader confidence regions indicate reduced accuracy. Nonetheless, the main track features remain.

Odoo CMS - a big picture

Figure 2: Track of a SeaTag-MOD ocean drifter processed with light and geomagnetic observations (left) and geomagnetic observations only (right). The tracks are surrounded by 50% (dark gray) and 95% (light gray) confidence regions. Beyond the reach of light, confidence declines but an approximate track is available.  Compare track colors for monthly progress. (Tag data courtesy Neil Hammerschlag, U Miami. Track processing courtesy CLS Track&Loc)

While not implemented in the firmware at this time, the tag's integrated accelerometer offers the theoretical capability to sense magnetic field inclination as well. In areas where the lines of equal magnetic field intensity and inclination run approximately perpendicular, such as in the South Atlantic or South Indian Ocean, there exists the opportunity to fix positions more precisely by the intersection of these two properties.

The accelerometer can also be used to sense activity patterns of the tagged animal.

Argos data transmission is powered by both the solar panel and the replaceable plug-in battery section. The solar panel affords opportunistic transmissions and Argos fixes during brief surfacing periods, as well as the transmission of lengthy datasets post pop-up.  The extended post-up transmission endurance, generally measured in months and at times exceeding a year, increases the opportunities for the physical recovery of a tag to obtain the full data set. However, the solar panel utility is limited by the identified vulnerabilities (see below).

Identification and Management of Tag Vulnerabilities

We have identified vulnerabilities in SeaTag-6K. First among them is the solar panel. During extreme and repeated pressure exposures,the solar panel has been observed to first loose its power generation capability and then its light sensing abilities. But, it can nonetheless support Argos transmissions until deep dives actually do occur, beyond which point the plug-in battery section provides energy for data collection and transmission. We found that the epoxy of the tag, even once hardened to a glass like consistency, will still flow slightly under extreme pressure. If pressure ramps are too fast (eg. 6000m to surface in a minute), small components may fail. A slower pressure ramp more akin to an actual dive reduces these stresses (compare slow and fast dives in figure 1). Nonetheless, readings from the small accelerometer deep inside the tag show that loading forces act throughout the tag. Consequently, a pressure cycle endurance limit may be expected.

To improve the probability of tagging success, SeaTag-6K includes specialized programming that transmits the essential statistics of both the deepest dive of the day and the record dive since mission start every tenth Argos transmission. Thus, the highest priority data is sent quickly, both during an opportunistic transmission or after pop-up. Transmission of daily summaries, histograms and time series data proceeds interleaved with the at the same time.

ST-GOLD Tag Replacement

Due to the higher anticipated risks of deep water tagging, SeaTag-6K a ST-GOLD tag replacement policy is incorporated in the SeaTag-6K price. This is available at two levels:

  • ST-GOLD-4K requires setting the crush depth release threshold to 4000m, limiting pressure exposure. This policy guarantees only post-tagging reporting with good depth data, i.e. survival of the Argos transmitter and depth sensor. It is suitable for vertical habitat studies.

  • ST-GOLD-6K sets the crush depth prevention threshold to 6000m. In addition to a reporting guarantee and depth sensor performance, it also covers magnetometer survival so that estimated tracks can be computed.

Either policy covers one replacement per tag, but can be renewed.

SeaTag-6K Specifications


PARAMETER SPECIFICATION
Size (w/ ST-MRPS-6k Battery, excl. antenna)

349 mm L x 25.5 mm (Ø tube) x 48 mm (Ø float)

Weight in air

317g

Flotation in sea water

53g

Maximum depth rating (design and test limit)

6842 m of sea water (10,000 PSI)

Power

Plug-in battery section and solar power

Battery endurance for data acquisition

See SeaTag-MOD battery endurance report

Sensors

Depth, temperature, light, three-axis magnetometer, three-axis accelerometer

Memory

2 GB (Approx. 62.5 million data packets of 32 bytes each)

Argos transmitted data packets (per user configuration)

Daily and overall record dive report, Daily summary report, depth histogram, high resolution acceleration (HRA) condensed time series, sensor snapshot report, engineering status report

Release mechanism

Kinetic (powder charged)

Re-usability

Via plug-in replacement of battery and release section